Catalytic cleaning processes can be used to partially or completely remove pollutants from the exhaust gas of an internal combustion engine. Specifically, exhaust gas cleaning may be achieved using a reducing agent that reduces one or more pollutants, for example, NOx, in the exhaust gas.
In order to reduce exhaust gas emissions from an internal combustion engine, the reducing agent (e.g. urea solution) can be sprayed into an exhaust passage using an injection device, as shown, for example, in WO2004111401. Typically, the activity of the reducing agent is triggered on contact with a catalyst downstream from the point of injection. The injection device is mounted to the passage of an exhaust system and is exposed to the hot gases passing through the system.
The general operation of a reagent dosing system is described with reference to FIG. 1. A diesel engine 1 produces an exhaust flow comprising various exhaust gases 3. The exhaust gases are conveyed through an exhaust system, indicated generally at 5, comprising an oxidation catalyst device 7, a selective reduction catalyst (SCR) device 9 and a slip catalyst 11.
The oxidation catalyst device 7 is a flow through device that consists of a canister containing a honeycomb-like structure or substrate. The substrate has a large surface area that is coated with an active catalyst layer. This layer contains a small, well dispersed amount of precious metals such as platinum or palladium. As the exhaust gases traverse the catalyst, carbon monoxide, gaseous hydrocarbons and liquid hydrocarbon particles (unburned fuel and oil) are oxidized, thereby reducing harmful emissions.
The SCR device 9 performs Selective Catalytic Reduction (SCR) of nitrogen oxide (NOx) using ammonia (derived from a source of urea) as a chemical reductant. The slip catalyst 11 is located downstream of the SCR device 9 to clean up any unreacted ammonia.
Urea for the SCR device 9 is stored in a tank 13 which is in fluid communication with the exhaust system 5. A pump 15 is provided to pump urea from the tank 13 to the exhaust system 5. The supply of urea is controlled by a control unit 17, for example the engine control unit, which receives engine speed and other engine parameters from the engine 1. An injection device 19 is used to inject the urea into the exhaust flow.
It is an object of the invention to provide an injection device which is convenient to manufacture and which is suitable for use in a reagent dosing system generally of the aforementioned type.